The invention relates to the regulation of laser diodes operating continuously and it finds an important although not exclusive application in the field of optical fiber digital transmissions.
It is known that laser diodes are widely used for optical fiber transmission, in particular because they are capable of operating at much higher frequencies than light emitting diodes. But on the other hand these laser diodes have the disadvantage of presenting considerable variations of the light power emitted in response to the small variation of the current with respect to the threshold value. Now, this threshold value presents appreciable variations which are either reversible, as a function of the temperature more especially, or irreversible following aging more particularly.
To palliate the effects of these variations, it is necessary to provide regulation of the output light power. Processes are already known for regulating the light power emitted by a laser diode under continuous operating conditions, in which an electric signal representative of said light power is compared with an electric reference signal corresponding to a reference power for elaborating an error signal, and the operating conditions of the laser diode are automatically modified as a function of said signal.
Usually, it is the electric current supplying the laser diode which is controlled by a feedback loop whose input value is formed by the difference between the measured output light power and a reference power.
It is frequently necessary to add to this servo control a temperature regulation for present laser diodes must operate at a temperature at most not much higher than the normal ambient temperature. High temperatures in fact accelerate the aging and increase the risk of destruction, whereas on the contrary lowering of the temperature during operation increases the lifespan. Another reason which makes a temperature regulation frequently necessary is the need to remove the heat released at the junction of the laser diode. This temperature regulation is generally effected by cooling with an active element, such as a Peltier effect element, controlled by a second feedback loop whose input signal is formed by the difference between the measured temperature of the laser and the reference temperature.
Different reasons, which will be given in detail further on, cause this solution to be unsatisfactory. In particular, this double regulation requires a larger number of components and it suffers from a defect in its very principle: to maintain the light power emitted, during aging, the supply current must be increased, which causes an increase in heating and in the power consumed by the Peltier effect element, resulting in an increase in the overall power consumed. In addition, it is difficult to measure the temperature of the junction of the laser diode, so that the temperature measured is that of the base of the diode, to which a thermistor is fixed. The temperature differences which exist between the base and the junction will increase gradually with increases of electric current during aging, so that, for a fixed reference temperature, the temperature of the junction increases, which further accelerates aging.
It is an object of the invention to provide an improved regulation process and device. It is a more specific object of the invention to provide a device which is simple in construction and which eliminates the above mentioned positive reaction phenomena.
To this end, the invention uses the fact that the threshold current of a laser diode increases with the temperature according to a law which is approximately exponential, and it proposes more especially a process of the above defined kind which is characterized in that the heat flow extracted from the laser diode is controlled so as to cancel out the error signal while feeding the laser diode with constant electric current.
The invention also proposes a regulation device for implementing the above defined process. This device comprises a photodetector disposed so as to supply an electric signal representative of the mean optical power of the laser diode and a circuit associated with the photodetector for supplying an error signal representative of the difference between the light power and a reference power, as well as a circuit for controlling the cooling means of the laser. It is more especially characterized in that the control circuit is mounted so as to receive the error signal and to control the cooling in a direction tending to cancel out the error signal and in that said supply circuit is a DC circuit.
Such an extremely simple device gives an entirely satisfactory result when the environment of the laser diode does not present sudden thermal shocks, which is for example the case in an undersea cable repeater. When the environment is on the contrary likely to present temperature variations whose rapidity is incompatible with the large time constant of the device such as defined above, which time constant is fixed more especially by the thermal inertia of the elements, the device is advantageously completed by addition of a servo control branch acting on the supply current of the laser diode. But it is essential to bear in mind that this branch has only an accessory role and only comes into play in a transitory fashion, during the time required for the thermal regulation to compensate for the effect of the temperature variation or in the case of failure of the main device.
The invention will be better understood from reading the following description of particular embodiments given by way of examples, and from the comparison which is made thereof with the prior art.